The present invention relates generally to the field of Selective Catalytic Reduction (SCR) gas inlet temperature control for boilers with a parallel convection back pass and, in particular, to a system and method for maintaining the combustion or flue gas entering the SCR system at or above a minimum injection temperature and minimum continuous operating temperature as specified by the supplier of the catalyst used in the system, even when operating the boiler at reduced loads.
Selective Catalytic Reduction (SCR) systems introduce ammonia into the flue gas upstream of a reactor filled with multiple blocks of catalyst, where nitrogen oxides (NOx) produced during combustion are reduced to nitrogen and water when combined with the ammonia on the active sites contained within the catalyst's micropore structure. System operation must proceed per the catalyst supplier's instructions; these instructions include limiting ammonia introduction into the flue gas only when the average flue gas temperature entering the SCR reactor meets or exceeds a minimum injection temperature for limited operation or a minimum continuous operating temperature for unlimited operation, up to the maximum allowable gas temperature. These minimum temperatures are set by the sulfur content of the fuel and the resulting expected sulfur trioxide (SO3) concentration in the products of combustion exiting the boiler economizer. Typically, the minimum injection temperature for limited operation is within a temperature range of about 520 degrees F. to about 620 degrees F., while the minimum continuous operating temperature for unlimited operation is within a temperature range of about 540 degrees F. to about 640 degrees F.
Typically, at a boiler or steam generator unit's maximum continuous rating (MCR), the flue gas temperature entering the SCR reactor meets or exceeds the catalyst supplier's minimum injection temperature and minimum continuous operating temperature. As boiler load decreases, the boiler exit gas temperature may fall to a temperature between the minimum injection temperature and minimum continuous operating temperature or even below the minimum injection temperature at varying loads depending on the fuel, firing method, and overall unit operation. For reactor inlet temperatures between the minimum injection temperature and minimum continuous operating temperature, ammonia injection may occur for only a limited time before the reagent must be shut off or gas temperature must be increased above the catalyst supplier's specified recovery temperature for an equivalent time that the unit was operated between the minimum injection temperature and minimum continuous operating temperature. If the average reactor inlet gas temperature falls below the minimum injection temperature, the reagent must be immediately shut off. In order to maintain the average reactor inlet gas temperature above the minimum injection temperature and minimum continuous operating temperature at lower boiler loads, the current industry practice has been to use an external economizer gas bypass. The external economizer gas bypass reroutes a portion of the hot gas exiting either the primary superheat or reheat section of the parallel convection back pass around the respective economizer heat transfer surface, where it is re-introduced into the main gas stream in order to maintain elevated gas temperatures entering the SCR reactor at reduced boiler loads.
SCRs can be applied to existing boilers or steam generators as a retrofit application, or they can be applied as part of new power plant installations. In some instances, the boiler/SCR arrangement has already been designed, and since many materials are already procured and fabricated, designers face the issue of limited space. This is typical of retrofit applications, except that on retrofits generally there is some freedom to relocate the SCR.
Conventional external boiler convection pass, gas by-pass systems are typically designed to make new penetrations in the casing of the boiler before and behind the convection pass tube banks that are intended to have the flue gas bypassed at reduced boiler loads. Typically this requires boiler casing penetrations, penetration seals, and gas flues external to the boiler setting that connect the take-off point to the desired downstream re-injection point of the boiler. Dampers, hangers, expansion joints, and structural steel used for support of the structure are also required for this conventional boiler convection pass heat transfer surface arrangement. There are undesirable aspects to this including boiler flyash buildup in the external bypass or “jumper” flues. In addition, there is the potential for leakage of the flue gas over time which reduces boiler heat transfer efficiency when the gas by-pass system is desired to be out of service and all the flue gas flow is desired to flow across the convection heat transfer surface at full load operation.
Additional details of SCR systems for NOx removal are provided in Chapter 34 of Steam/its generation and use, 41st Edition, Kitto and Stultz, Eds., Copyright © 2005, The Babcock & Wilcox Company, the text of which is hereby incorporated by reference as though fully set forth herein. Flue gas temperature control using conventional economizers are described in U.S. Pat. Nos. 7,021,248 to McNertney, Jr. et al. and 6,609,483 to Albrecht et al., the texts of which are hereby incorporated by reference as though fully set forth herein. Flue gas temperature control using an internal flue gas bypass are also described in U.S. Pat. Nos. 4,738,226 to Kashiwazaki et al. and 6,748,880 to DeSellem.